1. Field of the Invention
The present invention relates to a laser ionization sputtered neutral mass spectrometer in which a mass spectrometric analysis is carried out by determining a mass spectrum of a photoion formed by ionizing a neutral by UV laser rays among particles which are sputtered by irradiation of a solid sample, i.e., a substance to be analyzed, with an ion beam.
2. Description of the Prior Art
As typical methods for trace analysis of solid samples, there has been known the secondary ion mass spectrometry in which secondary ions sputtered from the surface of a sample through the irradiation with an ion beam are detected. However, since the secondary ion yield is low and a quantity of the secondary ions greatly varies depending on a kind of element, the quantity is not in proportion to the concentration of a specific element present in the sample and, therefore, this method suffers from a problem of precision from the viewpoint of quantitative analysis.
On the other hand, the quantity of the neutrals which are sputtered from the sample simultaneously with the secondary ions is in proportion to the concentration of the corresponding element present in the sample and, therefore, the sputtered neutral mass spectrometry in which neutrals are detected is an analytical method which can provide a high precision from the viewpoint of quantitative analysis. In particular, it has been known that the laser ionization sputtered neutral mass spectrometry in which neutrals are ionized by the irradiation with laser rays is a method capable of providing high ionization efficiency (see, for instance, C. H. Becker, J. Vac. Sci. Technol., 1987, A5, p. 1181).
However, the secondary ion mass spectrometry has a sensitivity in measurement more excellent than that achieved by the conventional sputtered neutral mass spectrometer, since the latter suffers from problems as will be discussed below. A measure for solving the foregoing problem is to simultaneously detect both neutrals and secondary ions, but the secondary ions cannot be detected with a high sensitivity by the conventional apparatuses. The outline of the conventional laser ionization sputtered neutral mass spectrometers will hereunder be described and the problems concerning the sensitivity in measurement, detection of secondary ions or the like thereof will be clarified below.
FIG. 1 shows an example of a conventional laser ionization sputtered neutral mass spectrometer. In FIG. 1, reference numeral 1 represents an ion source which generates an ion beam 2 through the ionization of a gas such as argon or oxygen or metal vapor. The ion beam 2 is converged by an electrostatic lens 3 and then pulsed by an ion-pulsing electrodes 4 to bombard the surface of a solid sample 5. Neutrals and secondary ions are discharged from the surface of the solid sample 5 through the bombardment with the sputter ion beam 2. The secondary ions 6 are extracted by an ion extraction electrode 7, but the neutrals 8 reach a photoionization region 9 at a velocity lower than that of the secondary ion 6, since they are not accelerated. In the photoionization region 9, the neutrals 8 are irradiated by UV laser rays 11 generated in a UV laser light source 10 and thus are photoionized to form photoions 12. The photoions 12 are extracted by the ion extraction electrodes 7, then passed through a time of flight type mass analyzer 13 and then converted into current signals in an ion detector 14. The current signals outputted from the ion detector 14 are detected as a current by a measuring instrument such as digital oscilloscope 15.
A first technique for detecting photoions comprises performing mass separation of photoions generated within a very short period of time. In this respect, a generation-time duration for the photoions 12 which are generated through the bombardment with the UV laser rays 11 is of the order of about several tens of nanoseconds. A time of flight type mass analyzer 13 is used for determining the quantity of the photoions 12 generated within such a short period of time. In such a time of flight type mass analyzer 13, the mass separation is performed by making the most use of the fact that among particles almost simultaneously generated, the lower the mass of particles, the shorter a time required for arriving at a detector, while the higher the mass of particles, the longer a time required for arriving at the detector.
A second technique for detecting photoions comprises separating the secondary ions 6 from the photoions 12. The secondary ions 6 discharged from the surface of the sample 5 interfere the detection of the photoions 12. The methods of this kind can be classified into two groups.
In the first method, an ion beam 2 is pulsed synchronously with laser rays 11 as shown in FIG. 1. Thus, pulsed secondary ions and pulsed neutrals are generated from the surface of the sample 5 by the action of the pulsed ion beam 2. The secondary ions 6 per se are accelerated towards the detector 14. On the other hand, the neutrals 8 move towards an ionization region while maintaining the initial velocity thereof and are accelerated only after the ionization by the irradiation with laser rays 11. For this reason, a difference in time required for arriving at the detector between the secondary ions and the neutrals arises. Thus, the detection of the secondary ions and the photoions can be performed, while making use of such a difference in the detection time.
The second method comprises accelerating the secondary ions by applying an energy greatly different from that for the photoions. There have been known a variety of such methods. For instance, as shown in FIG. 2, an electrode 16 is disposed between a sample 5 to be analyzed and a photoionization region 9 to thus cause repulsion of the second ions, thereby guiding only the neutrals into the ionization region 9.
A third technique for detecting the photoions is to use a means for detecting ions. The photoions are converted into a current by an ion detector and measured by a detector such as a digital oscilloscope.
As has been explained above, the conventional apparatuses principally comprises a time of flight type mass analyzer, a means for separating secondary ions and a detector which measures a quantity of electric current. However, the conventional apparatuses having such a construction suffer from the following problems when they are used in analysis requiring a high sensitivity. In the high sensitive analysis, it is necessary to carry out measurements over several times and to accumulate the data obtained, but the accumulated speed in the apparatus is very low, because the data outputted from the current detector such as a digital oscilloscope are two-dimensional data, i.e., a change in current with respect to time. Moreover, the conventional measuring instruments for detecting a current do not have a sufficient dynamic range for detecting an ion current originated from constituent elements of a sample to be analyzed and for detecting a quite low current derived from trace impurities and, therefore, cannot detect a quite low current. In addition, it is required to keep the photoionization region 9 away from the surface of the sample 5 to some extent in order to separate the secondary ions from the photoions, even if either of the methods for changing time and acceleration energy is adopted. This results in the reduction in a solid angle of photoionization and hence the reduction of an amount of neutrals to be ionized. For this reason, the sensitivity of these apparatuses is low and is of the order of ppm (see, for instance, C. H. Becker, J. Vac. Sci Technol., 1987, A5, p. 1181).
In respect of the determination of secondary ions, these apparatuses make it possible to detect the secondary ions. In this case, however, it is necessary to pulse the ion beam for sputtering the sample and the apparatuses are insufficient for use as a high sensitive secondary ion detector. As has been explained above, it has been difficult so far to carry out an analysis with a high sensitivity and an analysis of secondary ions when the conventional laser ionization sputtered neutral mass spectrometer is used.